Low sulfur recarburizer coke is a type of coke used in the production of high quality steels. Its purpose is to increase the carbon content of the steel without introducing any extraneous contaminants, especially sulfur and nitrogen. Historically, steel producers and recarburizer marketers have used crushed scrap graphite (graphitized premium coke) as the major source of recarburizer coke. However, this source has steadily declined as scrap rates in the graphite electrode production, and electric arc furnaces have been reduced. A market now exists for alternative sources of recarburizer coke with very low levels of sulfur and nitrogen.
Premium coke is a type of coke used for the manufacture of large graphite electrodes for use in electric arc furnaces employed in the steel industry. The quality of premium coke used in graphite electrodes is usually measured by its coefficient of thermal expansion (CTE) which may vary from as low as 0 to as high as 8 centimeters per centimeter per degree centigrade.times.10.sup.-7. Users of premium coke continuously seek graphite materials having lower CTE values.
Another type of coke which is manufactured in substantial quantities is so called aluminum grade coke, that is, coke which is used in manufacturing electrodes for use in the production of aluminum. This coke contains substantial amounts of sulfur and nitrogen which make it unsuitable for use as recarburizer coke. Also its CTE is substantially higher than that required for premium coke.
Each of the above types of cokes are obtained by subjecting heavy oils, usually petroleum oils, to delayed coking. However, the feedstocks used and the processing conditions used differ for each type of coke, thus they are usually manufactured in separate operations and from separate feedstocks.
In accordance with this invention, a virgin heavy oil is catalytically hydrotreated to substantially reduce its sulfur and nitrogen content; the hydrotreated product is separated into a light fraction and a heavy fraction, each of which is separately subjected to delayed coking; and the delayed coke is calcined to provide two products: a recarburizer or premium grade coke product containing not more than 0.1 weight percent sulfur; and a coke with higher levels of sulfur, which is suitable for use in the manufacture of electrodes used in the production of aluminum. The CTE of the recarburizer coke product is sufficiently low that this material after graphitization is suitable for use as premium coke.